Can exercise harm the baby?
There is no evidence that moderate exercise has any harmful effects on the unborn baby, or that it increases the risk of miscarriage, preterm labor or birth defects in a normal pregnancy.

In the past, there were few good studies on the effects of exercise on the baby. Providers had theoretical concerns that exercise could trigger preterm labor because it increases the levels of hormones that may stimulate uterine contractions. Recent studies have provided reassurance that exercise does not increase the risk for preterm labor in low-risk pregnancies.

Providers also were concerned that regular exercise could slow fetal growth because during exercise, blood tends to be diverted to the exercising muscles and, possibly, away from the uterus. Most studies have found that exercise has no effect on birthweight, and a few studies suggest that moderate exercise may actually increase birthweight. Recent studies have found that moderate exercise in early pregnancy improves growth of the placenta. The placenta supplies the baby with oxygen and nutrients, possibly contributing to an increase in birthweight.

Studies suggest that strenuous exercise, when continued through the third trimester of pregnancy, may slightly reduce birthweight. It does not, however, appear to increase a woman’s risk for having a low-birthweight baby (less than 5½ pounds). The weight difference appears to be mainly in body fat, and the babies remain in the normal range for weight.